#!/usr/bin/perl -l

# https://perlmonks.org/?node_id=1226058

use strict;
use warnings;

$_ = 'aabbcc';

my $len = length;
my $inner = join ',', split //, tr///csr;
my $glob = "{$inner}" x $len;

my $re = "^(?=.{$len}\$)";
while( /./ )
  {
  my $count = s/$&//g;
  $re .= "(?=(?:.*$&){$count})";
  }

$_ =~ $re and print for glob $glob;